Screening for congenital abnormalities has become a routine part of pregnancy care. Normal human development is hugely complex and while the vast majority of babies are born without birth defects, the unfortunate reality is that there is always a risk of abnormal development, regardless of a mother's or father's age, family or personal history, or lifestyle. Congenital abnormalities and birth defects affect 2-3% of all pregnancies, with most of these being minor. A small minority of babies have major birth defects.
Although some congenital conditions can be inherited, the vast majority of birth defects are random events that affect people who had thought they were low risk.
Ultrasound scanning has for many years been used alongside results from blood tests to assess the risk of a baby being affected by genetic conditions. Two pregnancy ultrasound scans are used to detect the vast majority of serious birth defects:
- 12 week nuchal translucency pregnancy scan This 'first trimester combined screen' is carried out between week 11 and 14. More information here.
- 20 week anatomy and anomaly pregnancy scan This 'second trimester scan' is carried out between 18 and 22 weeks. Follow this link to learn more.
The most widely known condition arising from a genetic abnormality is Down syndrome. This is one of a range of conditions known as 'trisomy' (where there are three copies of a chromosome instead of the usual two) that result in physical defects, some of which may be life threatening.
Please follow the links below if you would like more information on birth defects and screening in pregnancy.
Birth Defects with normal chromosomes
Most birth defects are sporadic types of birth defects that can involve any organ system: for example, the brain, spine, face, heart, kidneys, extremities, lungs or gastrointestinal tract. These may be either major or minor. In most cases, the cause of the defect cannot be identified and is not linked to abnormality in the chromosomes: it simply reflects the complexity of development from a single cell to a fully functioning person. Sometimes multiple anomalies may be present and fit the description of a "syndrome". A syndrome is a recognized pattern of birth defects.
A detailed fetal ultrasound scan, when performed by skilled personnel, can detect the vast majority of major birth defects. For example, high quality ultrasound can detect nearly all open spinal defects so a normal result virtually eliminates the possibility of a significant spinal defect. While major anomalies involving other organ systems can also generally be identified, some minor defects may remain undetected and certain types of anomalies can never be detected before birth.
Aneuploidy or Chromosome Abnormalities including Down syndrome
Chromosomes are the "blue prints" or "DNA" that make us what we are. Humans have 23 pairs of chromosomes, or 46 in total. A minority of birth defects are caused by chromosome abnormalities (aneuploidy) and while some of the physical defects may be treated after birth, the underlying chromosomal abnormality will always be present.
Chromosome abnormalities affect the number of chromosomes so there may be too many (the usual case) or too few. Trisomy 21 (Down syndrome) is one of the most common that is seen at birth. Affected individuals have 3 copies of the 21st chromosome, making a total of 47 chromosomes. Because chromosomes are in nearly every cell of the body, they can affect any organ system and the impact of Down syndrome can involve digestive problems, speech and other systems, and affected people tend to be mentally delayed.
In more serious chromosome abnormalities the effect on normal organ function is so severe that the baby may not survive the pregnancy, or is most likely to die shortly after birth. Trisomy 18 (Edwards syndrome) and Trisomy13 (Patau syndrome) are associated with a high rate of miscarriage. Babies with these conditions are typically affected by severe brain abnormalities and often have congenital heart defects as well as other birth defects. Most affected individuals die before or soon after birth and very few survive beyond the first year of life.
Screening for Birth Defects
The most effective way to screen for birth defects is to use ultrasound scanning in combination with a maternal blood test. Together, these offer a measurement of risk. Maternal age will also be taken into account (increased maternal age increases the risk of chromosome abnormalities) as well as personal and family history.
Your doctor will talk to you about your personal circumstances and your preferences. If you are in a high risk category you may choose to have further tests, such as the non-invasive prenatal screening tests Harmony Prenatal Test TM or MaterniT21 PLUS TM. The Medical Chambers Kensington is one of very few centres in the UK currently offering all three of these non-invasive tests.
The most accurate test is an invasive amniocentesis; but we recommend this is reserved for the unlikely situation that a non-invasive test reveals a very significant risk because the amniocentesis itself carries a very small risk of miscarriage.